/*
 * Copyright (c) 2012, 2013, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

/*
 * Clone of Java 8 SE java.util.Base64
 */
package com.ketayao.fensy.util;

import java.io.FilterOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.nio.ByteBuffer;
import java.nio.charset.Charset;
import java.util.Arrays;

/**
 * This class consists exclusively of static methods for obtaining encoders and
 * decoders for the Base64 encoding scheme. The implementation of this class
 * supports the following types of Base64 as specified in <a
 * href="http://www.ietf.org/rfc/rfc4648.txt">RFC 4648</a> and <a
 * href="http://www.ietf.org/rfc/rfc2045.txt">RFC 2045</a>.
 *
 * <ul>
 * <li><a name="basic"><b>Basic</b></a>
 * <p>
 * Uses "The Base64 Alphabet" as specified in Table 1 of RFC 4648 and RFC 2045
 * for encoding and decoding operation. The encoder does not add any line feed
 * (line separator) character. The decoder rejects data that contains characters
 * outside the base64 alphabet.
 * </p>
 * </li>
 *
 * <li><a name="url"><b>URL and Filename safe</b></a>
 * <p>
 * Uses the "URL and Filename safe Base64 Alphabet" as specified in Table 2 of
 * RFC 4648 for encoding and decoding. The encoder does not add any line feed
 * (line separator) character. The decoder rejects data that contains characters
 * outside the base64 alphabet.
 * </p>
 * </li>
 *
 * <li><a name="mime"><b>MIME</b></a>
 * <p>
 * Uses the "The Base64 Alphabet" as specified in Table 1 of RFC 2045 for
 * encoding and decoding operation. The encoded output must be represented in
 * lines of no more than 76 characters each and uses a carriage return
 * {@code '\r'} followed immediately by a linefeed {@code '\n'} as the line
 * separator. No line separator is added to the end of the encoded output. All
 * line separators or other characters not found in the base64 alphabet table
 * are ignored in decoding operation.
 * </p>
 * </li>
 * </ul>
 *
 * <p>
 * Unless otherwise noted, passing a {@code null} argument to a method of this
 * class will cause a {@link java.lang.NullPointerException
 * NullPointerException} to be thrown.
 *
 * @author Xueming Shen
 * @since 1.8
 */
public class Base64 {

    protected static final byte PAD_DEFAULT  = '=';                                                 // Allow static access to default

    /**
     * This array is a lookup table that translates Unicode characters drawn from the "Base64 Alphabet" (as specified
     * in Table 1 of RFC 2045) into their 6-bit positive integer equivalents. Characters that are not in the Base64
     * alphabet but fall within the bounds of the array are translated to -1.
     *
     * Note: '+' and '-' both decode to 62. '/' and '_' both decode to 63. This means decoder seamlessly handles both
     * URL_SAFE and STANDARD base64. (The encoder, on the other hand, needs to know ahead of time what to emit).
     *
     * Thanks to "commons" project in ws.apache.org for this code.
     * http://svn.apache.org/repos/asf/webservices/commons/trunk/modules/util/
     */
    private static final byte[] DECODE_TABLE = { -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
                                                 -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
                                                 -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
                                                 -1, -1, -1, -1, 62, -1, 62, -1, 63, 52, 53, 54, 55,
                                                 56, 57, 58, 59, 60, 61, -1, -1, -1, -1, -1, -1, -1,
                                                 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
                                                 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -1, -1,
                                                 -1, -1, 63, -1, 26, 27, 28, 29, 30, 31, 32, 33, 34,
                                                 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47,
                                                 48, 49, 50, 51 };

    private Base64() {
    }

    public static byte[] getBytesUtf8(String string) {
        return getBytes(string, Charset.forName("UTF-8"));
    }

    /**
     * Returns whether or not the <code>octet</code> is in the base 64 alphabet.
     *
     * @param octet
     *            The value to test
     * @return {@code true} if the value is defined in the the base 64 alphabet, {@code false} otherwise.
     * @since 1.4
     */
    public static boolean isBase64(byte octet) {
        return octet == PAD_DEFAULT
               || (octet >= 0 && octet < DECODE_TABLE.length && DECODE_TABLE[octet] != -1);
    }

    /**
     * Tests a given String to see if it contains only valid characters within the Base64 alphabet. Currently the
     * method treats whitespace as valid.
     *
     * @param base64
     *            String to test
     * @return {@code true} if all characters in the String are valid characters in the Base64 alphabet or if
     *         the String is empty; {@code false}, otherwise
     *  @since 1.5
     */
    public static boolean isBase64(String base64) {
        return isBase64(getBytesUtf8(base64));
    }

    /**
     * Tests a given byte array to see if it contains only valid characters within the Base64 alphabet. Currently the
     * method treats whitespace as valid.
     *
     * @param arrayOctet
     *            byte array to test
     * @return {@code true} if all bytes are valid characters in the Base64 alphabet or if the byte array is empty;
     *         {@code false}, otherwise
     * @since 1.5
     */
    public static boolean isBase64(byte[] arrayOctet) {
        for (int i = 0; i < arrayOctet.length; i++) {
            if (!isBase64(arrayOctet[i]) && !isWhiteSpace(arrayOctet[i])) {
                return false;
            }
        }
        return true;
    }

    /**
     * Checks if a byte value is whitespace or not.
     * Whitespace is taken to mean: space, tab, CR, LF
     * @param byteToCheck
     *            the byte to check
     * @return true if byte is whitespace, false otherwise
     */
    protected static boolean isWhiteSpace(byte byteToCheck) {
        switch (byteToCheck) {
            case ' ':
            case '\n':
            case '\r':
            case '\t':
                return true;
            default:
                return false;
        }
    }

    /**
     * Calls {@link String#getBytes(Charset)}
     *
     * @param string
     *            The string to encode (if null, return null).
     * @param charset
     *            The {@link Charset} to encode the {@code String}
     * @return the encoded bytes
     */
    private static byte[] getBytes(String string, Charset charset) {
        if (string == null) {
            return null;
        }
        return string.getBytes(charset);
    }

    /**
     * Returns a {@link Encoder} that encodes using the <a
     * href="#basic">Basic</a> type base64 encoding scheme.
     *
     * @return A Base64 encoder.
     */
    public static Encoder getEncoder() {
        return Encoder.RFC4648;
    }

    /**
     * Returns a {@link Encoder} that encodes using the <a href="#url">URL and
     * Filename safe</a> type base64 encoding scheme.
     *
     * @return A Base64 encoder.
     */
    public static Encoder getUrlEncoder() {
        return Encoder.RFC4648_URLSAFE;
    }

    /**
     * Returns a {@link Encoder} that encodes using the <a href="#mime">MIME</a>
     * type base64 encoding scheme.
     *
     * @return A Base64 encoder.
     */
    public static Encoder getMimeEncoder() {
        return Encoder.RFC2045;
    }

    /**
     * Returns a {@link Encoder} that encodes using the <a href="#mime">MIME</a>
     * type base64 encoding scheme with specified line length and line
     * separators.
     *
     * @param lineLength the length of each output line (rounded down to nearest
     * multiple of 4). If {@code lineLength <= 0} the output will not be
     * separated in lines
     * @param lineSeparator the line separator for each output line
     *
     * @return A Base64 encoder.
     *
     * @throws IllegalArgumentException if {@code lineSeparator} includes any
     * character of "The Base64 Alphabet" as specified in Table 1 of RFC 2045.
     */
    public static Encoder getMimeEncoder(int lineLength, byte[] lineSeparator) {
        if (lineSeparator == null) {
            throw new NullPointerException();
        }
        int[] base64 = Decoder.fromBase64;
        for (byte b : lineSeparator) {
            if (base64[b & 0xff] != -1) {
                throw new IllegalArgumentException(
                    "Illegal base64 line separator character 0x" + Integer.toString(b, 16));
            }
        }
        if (lineLength <= 0) {
            return Encoder.RFC4648;
        }
        return new Encoder(false, lineSeparator, lineLength >> 2 << 2, true);
    }

    /**
     * Returns a {@link Decoder} that decodes using the <a
     * href="#basic">Basic</a> type base64 encoding scheme.
     *
     * @return A Base64 decoder.
     */
    public static Decoder getDecoder() {
        return Decoder.RFC4648;
    }

    /**
     * Returns a {@link Decoder} that decodes using the <a href="#url">URL and
     * Filename safe</a> type base64 encoding scheme.
     *
     * @return A Base64 decoder.
     */
    public static Decoder getUrlDecoder() {
        return Decoder.RFC4648_URLSAFE;
    }

    /**
     * Returns a {@link Decoder} that decodes using the <a href="#mime">MIME</a>
     * type base64 decoding scheme.
     *
     * @return A Base64 decoder.
     */
    public static Decoder getMimeDecoder() {
        return Decoder.RFC2045;
    }

    /**
     * This class implements an encoder for encoding byte data using the Base64
     * encoding scheme as specified in RFC 4648 and RFC 2045.
     *
     * <p>
     * Instances of {@link Encoder} class are safe for use by multiple
     * concurrent threads.
     *
     * <p>
     * Unless otherwise noted, passing a {@code null} argument to a method of
     * this class will cause a {@link java.lang.NullPointerException
     * NullPointerException} to be thrown.
     *
     * @see Decoder
     * @since 1.8
     */
    public static class Encoder {

        private final byte[]  newline;
        private final int     linemax;
        private final boolean isURL;
        private final boolean doPadding;

        private Encoder(boolean isURL, byte[] newline, int linemax, boolean doPadding) {
            this.isURL = isURL;
            this.newline = newline;
            this.linemax = linemax;
            this.doPadding = doPadding;
        }

        /**
         * This array is a lookup table that translates 6-bit positive integer
         * index values into their "Base64 Alphabet" equivalents as specified in
         * "Table 1: The Base64 Alphabet" of RFC 2045 (and RFC 4648).
         */
        private static final char[] toBase64        = { 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I',
                                                        'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R',
                                                        'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z', 'a',
                                                        'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j',
                                                        'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's',
                                                        't', 'u', 'v', 'w', 'x', 'y', 'z', '0', '1',
                                                        '2', '3', '4', '5', '6', '7', '8', '9', '+',
                                                        '/' };

        /**
         * It's the lookup table for "URL and Filename safe Base64" as specified
         * in Table 2 of the RFC 4648, with the '+' and '/' changed to '-' and
         * '_'. This table is used when BASE64_URL is specified.
         */
        private static final char[] toBase64URL     = { 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I',
                                                        'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R',
                                                        'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z', 'a',
                                                        'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j',
                                                        'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's',
                                                        't', 'u', 'v', 'w', 'x', 'y', 'z', '0', '1',
                                                        '2', '3', '4', '5', '6', '7', '8', '9', '-',
                                                        '_' };

        private static final int    MIMELINEMAX     = 76;
        private static final byte[] CRLF            = new byte[] { '\r', '\n' };

        static final Encoder        RFC4648         = new Encoder(false, null, -1, true);
        static final Encoder        RFC4648_URLSAFE = new Encoder(true, null, -1, true);
        static final Encoder        RFC2045         = new Encoder(false, CRLF, MIMELINEMAX, true);

        private final int outLength(int srclen) {
            int len = 0;
            if (doPadding) {
                len = 4 * ((srclen + 2) / 3);
            } else {
                int n = srclen % 3;
                len = 4 * (srclen / 3) + (n == 0 ? 0 : n + 1);
            }
            if (linemax > 0) // line separators
            {
                len += (len - 1) / linemax * newline.length;
            }
            return len;
        }

        /**
         * Encodes all bytes from the specified byte array into a
         * newly-allocated byte array using the {@link Base64} encoding scheme.
         * The returned byte array is of the length of the resulting bytes.
         *
         * @param src the byte array to encode
         * @return A newly-allocated byte array containing the resulting encoded
         * bytes.
         */
        public byte[] encode(byte[] src) {
            int len = outLength(src.length); // dst array size
            byte[] dst = new byte[len];
            int ret = encode0(src, 0, src.length, dst);
            if (ret != dst.length) {
                return Arrays.copyOf(dst, ret);
            }
            return dst;
        }

        /**
         * Encodes all bytes from the specified byte array using the
         * {@link Base64} encoding scheme, writing the resulting bytes to the
         * given output byte array, starting at offset 0.
         *
         * <p>
         * It is the responsibility of the invoker of this method to make sure
         * the output byte array {@code dst} has enough space for encoding all
         * bytes from the input byte array. No bytes will be written to the
         * output byte array if the output byte array is not big enough.
         *
         * @param src the byte array to encode
         * @param dst the output byte array
         * @return The number of bytes written to the output byte array
         *
         * @throws IllegalArgumentException if {@code dst} does not have enough
         * space for encoding all input bytes.
         */
        public int encode(byte[] src, byte[] dst) {
            int len = outLength(src.length); // dst array size
            if (dst.length < len) {
                throw new IllegalArgumentException(
                    "Output byte array is too small for encoding all input bytes");
            }
            return encode0(src, 0, src.length, dst);
        }

        /**
         * Encodes the specified byte array into a String using the
         * {@link Base64} encoding scheme.
         *
         * <p>
         * This method first encodes all input bytes into a base64 encoded byte
         * array and then constructs a new String by using the encoded byte
         * array and the {@link java.nio.charset.StandardCharsets#ISO_8859_1
         * ISO-8859-1} charset.
         *
         * <p>
         * In other words, an invocation of this method has exactly the same
         * effect as invoking
         * {@code new String(encode(src), StandardCharsets.ISO_8859_1)}.
         *
         * @param src the byte array to encode
         * @return A String containing the resulting Base64 encoded characters
         */
        @SuppressWarnings("deprecation")
        public String encodeToString(byte[] src) {
            byte[] encoded = encode(src);
            return new String(encoded, 0, 0, encoded.length);
        }

        /**
         * Encodes all remaining bytes from the specified byte buffer into a
         * newly-allocated ByteBuffer using the {@link Base64} encoding scheme.
         *
         * Upon return, the source buffer's position will be updated to its
         * limit; its limit will not have been changed. The returned output
         * buffer's position will be zero and its limit will be the number of
         * resulting encoded bytes.
         *
         * @param buffer the source ByteBuffer to encode
         * @return A newly-allocated byte buffer containing the encoded bytes.
         */
        public ByteBuffer encode(ByteBuffer buffer) {
            int len = outLength(buffer.remaining());
            byte[] dst = new byte[len];
            int ret = 0;
            if (buffer.hasArray()) {
                ret = encode0(buffer.array(), buffer.arrayOffset() + buffer.position(),
                    buffer.arrayOffset() + buffer.limit(), dst);
                buffer.position(buffer.limit());
            } else {
                byte[] src = new byte[buffer.remaining()];
                buffer.get(src);
                ret = encode0(src, 0, src.length, dst);
            }
            if (ret != dst.length) {
                dst = Arrays.copyOf(dst, ret);
            }
            return ByteBuffer.wrap(dst);
        }

        /**
         * Wraps an output stream for encoding byte data using the
         * {@link Base64} encoding scheme.
         *
         * <p>
         * It is recommended to promptly close the returned output stream after
         * use, during which it will flush all possible leftover bytes to the
         * underlying output stream. Closing the returned output stream will
         * close the underlying output stream.
         *
         * @param os the output stream.
         * @return the output stream for encoding the byte data into the
         * specified Base64 encoded format
         */
        public OutputStream wrap(OutputStream os) {
            if (os == null) {
                throw new NullPointerException();
            }
            return new EncOutputStream(os, isURL ? toBase64URL : toBase64, newline, linemax,
                doPadding);
        }

        /**
         * Returns an encoder instance that encodes equivalently to this one,
         * but without adding any padding character at the end of the encoded
         * byte data.
         *
         * <p>
         * The encoding scheme of this encoder instance is unaffected by this
         * invocation. The returned encoder instance should be used for
         * non-padding encoding operation.
         *
         * @return an equivalent encoder that encodes without adding any padding
         * character at the end
         */
        public Encoder withoutPadding() {
            if (!doPadding) {
                return this;
            }
            return new Encoder(isURL, newline, linemax, false);
        }

        private int encode0(byte[] src, int off, int end, byte[] dst) {
            char[] base64 = isURL ? toBase64URL : toBase64;
            int sp = off;
            int slen = (end - off) / 3 * 3;
            int sl = off + slen;
            if (linemax > 0 && slen > linemax / 4 * 3) {
                slen = linemax / 4 * 3;
            }
            int dp = 0;
            while (sp < sl) {
                int sl0 = Math.min(sp + slen, sl);
                for (int sp0 = sp, dp0 = dp; sp0 < sl0;) {
                    int bits = (src[sp0++] & 0xff) << 16 | (src[sp0++] & 0xff) << 8
                               | (src[sp0++] & 0xff);
                    dst[dp0++] = (byte) base64[(bits >>> 18) & 0x3f];
                    dst[dp0++] = (byte) base64[(bits >>> 12) & 0x3f];
                    dst[dp0++] = (byte) base64[(bits >>> 6) & 0x3f];
                    dst[dp0++] = (byte) base64[bits & 0x3f];
                }
                int dlen = (sl0 - sp) / 3 * 4;
                dp += dlen;
                sp = sl0;
                if (dlen == linemax && sp < end) {
                    for (byte b : newline) {
                        dst[dp++] = b;
                    }
                }
            }
            if (sp < end) { // 1 or 2 leftover bytes
                int b0 = src[sp++] & 0xff;
                dst[dp++] = (byte) base64[b0 >> 2];
                if (sp == end) {
                    dst[dp++] = (byte) base64[(b0 << 4) & 0x3f];
                    if (doPadding) {
                        dst[dp++] = '=';
                        dst[dp++] = '=';
                    }
                } else {
                    int b1 = src[sp++] & 0xff;
                    dst[dp++] = (byte) base64[(b0 << 4) & 0x3f | (b1 >> 4)];
                    dst[dp++] = (byte) base64[(b1 << 2) & 0x3f];
                    if (doPadding) {
                        dst[dp++] = '=';
                    }
                }
            }
            return dp;
        }
    }

    /**
     * This class implements a decoder for decoding byte data using the Base64
     * encoding scheme as specified in RFC 4648 and RFC 2045.
     *
     * <p>
     * The Base64 padding character {@code '='} is accepted and interpreted as
     * the end of the encoded byte data, but is not required. So if the final
     * unit of the encoded byte data only has two or three Base64 characters
     * (without the corresponding padding character(s) padded), they are decoded
     * as if followed by padding character(s). If there is a padding character
     * present in the final unit, the correct number of padding character(s)
     * must be present, otherwise {@code IllegalArgumentException} (
     * {@code IOException} when reading from a Base64 stream) is thrown during
     * decoding.
     *
     * <p>
     * Instances of {@link Decoder} class are safe for use by multiple
     * concurrent threads.
     *
     * <p>
     * Unless otherwise noted, passing a {@code null} argument to a method of
     * this class will cause a {@link java.lang.NullPointerException
     * NullPointerException} to be thrown.
     *
     * @see Encoder
     * @since 1.8
     */
    public static class Decoder {

        private final boolean isURL;
        private final boolean isMIME;

        private Decoder(boolean isURL, boolean isMIME) {
            this.isURL = isURL;
            this.isMIME = isMIME;
        }

        /**
         * Lookup table for decoding unicode characters drawn from the "Base64
         * Alphabet" (as specified in Table 1 of RFC 2045) into their 6-bit
         * positive integer equivalents. Characters that are not in the Base64
         * alphabet but fall within the bounds of the array are encoded to -1.
         *
         */
        private static final int[] fromBase64 = new int[256];

        static {
            Arrays.fill(fromBase64, -1);
            for (int i = 0; i < Encoder.toBase64.length; i++) {
                fromBase64[Encoder.toBase64[i]] = i;
            }
            fromBase64['='] = -2;
        }

        /**
         * Lookup table for decoding "URL and Filename safe Base64 Alphabet" as
         * specified in Table2 of the RFC 4648.
         */
        private static final int[] fromBase64URL = new int[256];

        static {
            Arrays.fill(fromBase64URL, -1);
            for (int i = 0; i < Encoder.toBase64URL.length; i++) {
                fromBase64URL[Encoder.toBase64URL[i]] = i;
            }
            fromBase64URL['='] = -2;
        }

        static final Decoder RFC4648         = new Decoder(false, false);
        static final Decoder RFC4648_URLSAFE = new Decoder(true, false);
        static final Decoder RFC2045         = new Decoder(false, true);

        /**
         * Decodes all bytes from the input byte array using the {@link Base64}
         * encoding scheme, writing the results into a newly-allocated output
         * byte array. The returned byte array is of the length of the resulting
         * bytes.
         *
         * @param src the byte array to decode
         *
         * @return A newly-allocated byte array containing the decoded bytes.
         *
         * @throws IllegalArgumentException if {@code src} is not in valid
         * Base64 scheme
         */
        public byte[] decode(byte[] src) {
            byte[] dst = new byte[outLength(src, 0, src.length)];
            int ret = decode0(src, 0, src.length, dst);
            if (ret != dst.length) {
                dst = Arrays.copyOf(dst, ret);
            }
            return dst;
        }

        /**
         * Decodes a Base64 encoded String into a newly-allocated byte array
         * using the {@link Base64} encoding scheme.
         *
         * <p>
         * An invocation of this method has exactly the same effect as invoking
         * {@code decode(src.getBytes(StandardCharsets.ISO_8859_1))}
         *
         * @param src the string to decode
         *
         * @return A newly-allocated byte array containing the decoded bytes.
         *
         * @throws IllegalArgumentException if {@code src} is not in valid
         * Base64 scheme
         */
        public byte[] decode(String src) {
            return decode(src.getBytes(Charset.forName("ISO-8859-1")));
        }

        /**
         * Decodes all bytes from the input byte array using the {@link Base64}
         * encoding scheme, writing the results into the given output byte
         * array, starting at offset 0.
         *
         * <p>
         * It is the responsibility of the invoker of this method to make sure
         * the output byte array {@code dst} has enough space for decoding all
         * bytes from the input byte array. No bytes will be be written to the
         * output byte array if the output byte array is not big enough.
         *
         * <p>
         * If the input byte array is not in valid Base64 encoding scheme then
         * some bytes may have been written to the output byte array before
         * IllegalargumentException is thrown.
         *
         * @param src the byte array to decode
         * @param dst the output byte array
         *
         * @return The number of bytes written to the output byte array
         *
         * @throws IllegalArgumentException if {@code src} is not in valid
         * Base64 scheme, or {@code dst} does not have enough space for decoding
         * all input bytes.
         */
        public int decode(byte[] src, byte[] dst) {
            int len = outLength(src, 0, src.length);
            if (dst.length < len) {
                throw new IllegalArgumentException(
                    "Output byte array is too small for decoding all input bytes");
            }
            return decode0(src, 0, src.length, dst);
        }

        /**
         * Decodes all bytes from the input byte buffer using the {@link Base64}
         * encoding scheme, writing the results into a newly-allocated
         * ByteBuffer.
         *
         * <p>
         * Upon return, the source buffer's position will be updated to its
         * limit; its limit will not have been changed. The returned output
         * buffer's position will be zero and its limit will be the number of
         * resulting decoded bytes
         *
         * <p>
         * {@code IllegalArgumentException} is thrown if the input buffer is not
         * in valid Base64 encoding scheme. The position of the input buffer
         * will not be advanced in this case.
         *
         * @param buffer the ByteBuffer to decode
         *
         * @return A newly-allocated byte buffer containing the decoded bytes
         *
         * @throws IllegalArgumentException if {@code src} is not in valid
         * Base64 scheme.
         */
        public ByteBuffer decode(ByteBuffer buffer) {
            int pos0 = buffer.position();
            try {
                byte[] src;
                int sp, sl;
                if (buffer.hasArray()) {
                    src = buffer.array();
                    sp = buffer.arrayOffset() + buffer.position();
                    sl = buffer.arrayOffset() + buffer.limit();
                    buffer.position(buffer.limit());
                } else {
                    src = new byte[buffer.remaining()];
                    buffer.get(src);
                    sp = 0;
                    sl = src.length;
                }
                byte[] dst = new byte[outLength(src, sp, sl)];
                return ByteBuffer.wrap(dst, 0, decode0(src, sp, sl, dst));
            } catch (IllegalArgumentException iae) {
                buffer.position(pos0);
                throw iae;
            }
        }

        /**
         * Returns an input stream for decoding {@link Base64} encoded byte
         * stream.
         *
         * <p>
         * The {@code read} methods of the returned {@code InputStream} will
         * throw {@code IOException} when reading bytes that cannot be decoded.
         *
         * <p>
         * Closing the returned input stream will close the underlying input
         * stream.
         *
         * @param is the input stream
         *
         * @return the input stream for decoding the specified Base64 encoded
         * byte stream
         */
        public InputStream wrap(InputStream is) {
            if (is == null) {
                throw new NullPointerException();
            }
            return new DecInputStream(is, isURL ? fromBase64URL : fromBase64, isMIME);
        }

        private int outLength(byte[] src, int sp, int sl) {
            int[] base64 = isURL ? fromBase64URL : fromBase64;
            int paddings = 0;
            int len = sl - sp;
            if (len == 0) {
                return 0;
            }
            if (len < 2) {
                if (isMIME && base64[0] == -1) {
                    return 0;
                }
                throw new IllegalArgumentException(
                    "Input byte[] should at least have 2 bytes for base64 bytes");
            }
            if (isMIME) {
                // scan all bytes to fill out all non-alphabet. a performance
                // trade-off of pre-scan or Arrays.copyOf
                int n = 0;
                while (sp < sl) {
                    int b = src[sp++] & 0xff;
                    if (b == '=') {
                        len -= (sl - sp + 1);
                        break;
                    }
                    if ((b = base64[b]) == -1) {
                        n++;
                    }
                }
                len -= n;
            } else {
                if (src[sl - 1] == '=') {
                    paddings++;
                    if (src[sl - 2] == '=') {
                        paddings++;
                    }
                }
            }
            if (paddings == 0 && (len & 0x3) != 0) {
                paddings = 4 - (len & 0x3);
            }
            return 3 * ((len + 3) / 4) - paddings;
        }

        private int decode0(byte[] src, int sp, int sl, byte[] dst) {
            int[] base64 = isURL ? fromBase64URL : fromBase64;
            int dp = 0;
            int bits = 0;
            int shiftto = 18; // pos of first byte of 4-byte atom
            while (sp < sl) {
                int b = src[sp++] & 0xff;
                if ((b = base64[b]) < 0) {
                    if (b == -2) { // padding byte '='
                        // = shiftto==18 unnecessary padding
                        // x= shiftto==12 a dangling single x
                        // x to be handled together with non-padding case
                        // xx= shiftto==6&&sp==sl missing last =
                        // xx=y shiftto==6 last is not =
                        if (shiftto == 6 && (sp == sl || src[sp++] != '=') || shiftto == 18) {
                            throw new IllegalArgumentException(
                                "Input byte array has wrong 4-byte ending unit");
                        }
                        break;
                    }
                    if (isMIME) // skip if for rfc2045
                    {
                        continue;
                    } else {
                        throw new IllegalArgumentException(
                            "Illegal base64 character " + Integer.toString(src[sp - 1], 16));
                    }
                }
                bits |= (b << shiftto);
                shiftto -= 6;
                if (shiftto < 0) {
                    dst[dp++] = (byte) (bits >> 16);
                    dst[dp++] = (byte) (bits >> 8);
                    dst[dp++] = (byte) (bits);
                    shiftto = 18;
                    bits = 0;
                }
            }
            // reached end of byte array or hit padding '=' characters.
            if (shiftto == 6) {
                dst[dp++] = (byte) (bits >> 16);
            } else if (shiftto == 0) {
                dst[dp++] = (byte) (bits >> 16);
                dst[dp++] = (byte) (bits >> 8);
            } else if (shiftto == 12) {
                // dangling single "x", incorrectly encoded.
                throw new IllegalArgumentException("Last unit does not have enough valid bits");
            }
            // anything left is invalid, if is not MIME.
            // if MIME, ignore all non-base64 character
            while (sp < sl) {
                if (isMIME && base64[src[sp++]] < 0) {
                    continue;
                }
                throw new IllegalArgumentException(
                    "Input byte array has incorrect ending byte at " + sp);
            }
            return dp;
        }
    }

    /*
     * An output stream for encoding bytes into the Base64.
     */
    private static class EncOutputStream extends FilterOutputStream {

        private int           leftover = 0;
        private int           b0, b1, b2;
        private boolean       closed   = false;

        private final char[]  base64;          // byte->base64 mapping
        private final byte[]  newline;         // line separator, if needed
        private final int     linemax;
        private final boolean doPadding;       // whether or not to pad
        private int           linepos  = 0;

        EncOutputStream(OutputStream os, char[] base64, byte[] newline, int linemax,
                        boolean doPadding) {
            super(os);
            this.base64 = base64;
            this.newline = newline;
            this.linemax = linemax;
            this.doPadding = doPadding;
        }

        @Override
        public void write(int b) throws IOException {
            byte[] buf = new byte[1];
            buf[0] = (byte) (b & 0xff);
            write(buf, 0, 1);
        }

        private void checkNewline() throws IOException {
            if (linepos == linemax) {
                out.write(newline);
                linepos = 0;
            }
        }

        @Override
        public void write(byte[] b, int off, int len) throws IOException {
            if (closed) {
                throw new IOException("Stream is closed");
            }
            if (off < 0 || len < 0 || off + len > b.length) {
                throw new ArrayIndexOutOfBoundsException();
            }
            if (len == 0) {
                return;
            }
            if (leftover != 0) {
                if (leftover == 1) {
                    b1 = b[off++] & 0xff;
                    len--;
                    if (len == 0) {
                        leftover++;
                        return;
                    }
                }
                b2 = b[off++] & 0xff;
                len--;
                checkNewline();
                out.write(base64[b0 >> 2]);
                out.write(base64[(b0 << 4) & 0x3f | (b1 >> 4)]);
                out.write(base64[(b1 << 2) & 0x3f | (b2 >> 6)]);
                out.write(base64[b2 & 0x3f]);
                linepos += 4;
            }
            int nBits24 = len / 3;
            leftover = len - (nBits24 * 3);
            while (nBits24-- > 0) {
                checkNewline();
                int bits = (b[off++] & 0xff) << 16 | (b[off++] & 0xff) << 8 | (b[off++] & 0xff);
                out.write(base64[(bits >>> 18) & 0x3f]);
                out.write(base64[(bits >>> 12) & 0x3f]);
                out.write(base64[(bits >>> 6) & 0x3f]);
                out.write(base64[bits & 0x3f]);
                linepos += 4;
            }
            if (leftover == 1) {
                b0 = b[off++] & 0xff;
            } else if (leftover == 2) {
                b0 = b[off++] & 0xff;
                b1 = b[off++] & 0xff;
            }
        }

        @Override
        public void close() throws IOException {
            if (!closed) {
                closed = true;
                if (leftover == 1) {
                    checkNewline();
                    out.write(base64[b0 >> 2]);
                    out.write(base64[(b0 << 4) & 0x3f]);
                    if (doPadding) {
                        out.write('=');
                        out.write('=');
                    }
                } else if (leftover == 2) {
                    checkNewline();
                    out.write(base64[b0 >> 2]);
                    out.write(base64[(b0 << 4) & 0x3f | (b1 >> 4)]);
                    out.write(base64[(b1 << 2) & 0x3f]);
                    if (doPadding) {
                        out.write('=');
                    }
                }
                leftover = 0;
                out.close();
            }
        }
    }

    /*
     * An input stream for decoding Base64 bytes
     */
    private static class DecInputStream extends InputStream {

        private final InputStream is;
        private final boolean     isMIME;
        private final int[]       base64;         // base64 -> byte mapping
        private int               bits    = 0;    // 24-bit buffer for decoding
        private int               nextin  = 18;   // next available "off" in "bits" for input;
        // -> 18, 12, 6, 0
        private int               nextout = -8;   // next available "off" in "bits" for output;
        // -> 8, 0, -8 (no byte for output)
        private boolean           eof     = false;
        private boolean           closed  = false;

        DecInputStream(InputStream is, int[] base64, boolean isMIME) {
            this.is = is;
            this.base64 = base64;
            this.isMIME = isMIME;
        }

        private byte[] sbBuf = new byte[1];

        @Override
        public int read() throws IOException {
            return read(sbBuf, 0, 1) == -1 ? -1 : sbBuf[0] & 0xff;
        }

        @Override
        public int read(byte[] b, int off, int len) throws IOException {
            if (closed) {
                throw new IOException("Stream is closed");
            }
            if (eof && nextout < 0) // eof and no leftover
            {
                return -1;
            }
            if (off < 0 || len < 0 || len > b.length - off) {
                throw new IndexOutOfBoundsException();
            }
            int oldOff = off;
            if (nextout >= 0) { // leftover output byte(s) in bits buf
                do {
                    if (len == 0) {
                        return off - oldOff;
                    }
                    b[off++] = (byte) (bits >> nextout);
                    len--;
                    nextout -= 8;
                } while (nextout >= 0);
                bits = 0;
            }
            while (len > 0) {
                int v = is.read();
                if (v == -1) {
                    eof = true;
                    if (nextin != 18) {
                        if (nextin == 12) {
                            throw new IOException(
                                "Base64 stream has one un-decoded dangling byte.");
                        }
                        // treat ending xx/xxx without padding character legal.
                        // same logic as v == '=' below
                        b[off++] = (byte) (bits >> (16));
                        len--;
                        if (nextin == 0) { // only one padding byte
                            if (len == 0) { // no enough output space
                                bits >>= 8; // shift to lowest byte
                                nextout = 0;
                            } else {
                                b[off++] = (byte) (bits >> 8);
                            }
                        }
                    }
                    if (off == oldOff) {
                        return -1;
                    } else {
                        return off - oldOff;
                    }
                }
                if (v == '=') { // padding byte(s)
                    // = shiftto==18 unnecessary padding
                    // x= shiftto==12 dangling x, invalid unit
                    // xx= shiftto==6 && missing last '='
                    // xx=y or last is not '='
                    if (nextin == 18 || nextin == 12 || nextin == 6 && is.read() != '=') {
                        throw new IOException("Illegal base64 ending sequence:" + nextin);
                    }
                    b[off++] = (byte) (bits >> (16));
                    len--;
                    if (nextin == 0) { // only one padding byte
                        if (len == 0) { // no enough output space
                            bits >>= 8; // shift to lowest byte
                            nextout = 0;
                        } else {
                            b[off++] = (byte) (bits >> 8);
                        }
                    }
                    eof = true;
                    break;
                }
                if ((v = base64[v]) == -1) {
                    if (isMIME) // skip if for rfc2045
                    {
                        continue;
                    } else {
                        throw new IOException(
                            "Illegal base64 character " + Integer.toString(v, 16));
                    }
                }
                bits |= (v << nextin);
                if (nextin == 0) {
                    nextin = 18; // clear for next
                    nextout = 16;
                    while (nextout >= 0) {
                        b[off++] = (byte) (bits >> nextout);
                        len--;
                        nextout -= 8;
                        if (len == 0 && nextout >= 0) { // don't clean "bits"
                            return off - oldOff;
                        }
                    }
                    bits = 0;
                } else {
                    nextin -= 6;
                }
            }
            return off - oldOff;
        }

        @Override
        public int available() throws IOException {
            if (closed) {
                throw new IOException("Stream is closed");
            }
            return is.available(); // TBD:
        }

        @Override
        public void close() throws IOException {
            if (!closed) {
                closed = true;
                is.close();
            }
        }
    }
}
